BOTTOM ASH

Bottom ash is formed in coal furnaces. It is made from agglomerated ash particles that
are too large to be carried in the flue gases and fall through open
grates to an ash hopper at the bottom of the furnace.

Before it is fed into the boiler, coal is pulverized to approximately the size of baking flour. The carbon and other combustible matter in the fine coal is burned, leaving the non-combustible ash behind. Because the temperatures in the boiler are very high (around 1500-1700 degrees C, for bituminous coal, 1300-1600 for lower rank coal.), the ash is softened and melted in the hot zones. Swirling air moves the melted ash out of the higher-temperature zones where it cools into mostly solid spherical particles, which are swept out of the boiler as fly ash. Some of the melted ash (10-20%) accumulates on the boiler walls and against steam tubes in the boiler and forms solidified masses sometimes called ‘clinker’. The clinker eventually builds up and is either blown off by jets of air or simply falls to the bottom of the boiler where it is removed -- hence the term 'bottom ash.'

Since the fused bottom ash is still hot, when it is removed from the boiler, it falls into a water bath or quench tank where it is cooled before passing through a clinker grinder, which reduces the size of any large chunks to smaller than approximately 2 inches. The crushed bottom ash is then removed for storage, disposal or use.

Bottom ash is primarily comprised of fused coarser ash particles. Frequently, these particles are quite porous and look like volcanic lava. If the bottom ash is competent, it can be used as lightweight aggregate, an important component in making concrete blocks. Since the density of some bottom ash is less than half that of conventional aggregate, the concrete blocks will be much lighter and just as strong. If the bottom ash is too crumbly or 'friable,' it is too weak to be used in blocks.

Some bottom ashes, particularly those derived from high-sulfur coals and lower rank coals, are not very porous and are quite dense. While these cannot be used to make lightweight blocks, they can be used in other applications where conventional aggregates are used.

One of the most common uses for bottom ash is as structural fill. At many construction sites, large quantities of fill are frequently required to level low places for construction and drainage purposes, build embankments, fill trenches, backfill foundations, etc. The fill is typically local aggregate or crushed stone. Bottom ash is commonly used in these applications if it is available and close enough to be transported economically.

HOW IS BOTTOM ASH CREATED?

PRODUCTS

"Coal combustion products are the solid residues generated by coal-burning electric utilities in the production of electricity. In 1998, electricity accounted for about 35% of the primary energy use in the United States and was produced by electric power generators designed to convert different fuel types into electricity. More than one-half of the electricity in the United States was generated by burning coal. (Courtesy of USGS)".